The peptidoglycan layer (PG) is an essential component of the bacterial cell envelope that protects the cell from osmotic lysis. This structure is unique to the bacterial kingdom, making it a valuable target of many antibiotics. Penicillin binding protein 1a (PBP1a) and penicillin binding protein 1b (PBP1b) are thought to be the primary enzymes responsible for PG synthesis, because their simultaneous inactivation leads to lysis.11 These two enzymes synthesize the PG layer by polymerizing glycan strands and then cross-linking these strands into the existing PG layer. Current work in the Bernhardt lab suggests that certain enzymes known as lytic transglycosylases (LTs) may act in conjunction with the penicillin binding proteins (PBPs), however the specifics of this interaction are not yet elucidated. Interestingly, LTs cleave glycan strands, so we predict that cells must possess a way to coordinate the activity of PBPs with the antagonistic activity of LTs. Recently, the Escherichia coli outer membrane lipoproteins LpoA and LpoB were discovered to be required for the in vivo activity of PBP1a and PBP1b, respectively.11, 12 However, the purpose of having activators of a constitutive process is unclear. This project aims to gain further understanding of the regulation of PG synthesis by determining the physiological role of LpoA and LpoB. To accomplish this, I will identify additional regulatory targets of LpoA and LpoB, identify suppressors that overcome the cell wall defects of strains containing a constitutively active PBP1b allele, and determine the physiological function of LpoA and the mechanism of its activation of PBP1a. The proposed experiments will build on the strengths of the Bernhardt lab in bacterial cell envelope biology and have the potential to reveal novel biological mechanisms relevant to future antibiotic development.